This application has relation to U.S. patent applications Ser. Nos. 07/795,989 (filed Nov. 22, 1991), 07/850,283 (filed Mar. 12, 1992), 07/878,469 (filed May 5, 1992), 07/939,600 (filed Sep. 2, 1992), and 07/969,072 (filed Oct. 30, 1992).
1. Field of the Invention
The present invention relates in general to automotive automatic transmissions, and more particularly to a control system for controlling the automotive automatic transmissions.
2. Description of the Prior Art
In order to improve the transmission performance of automotive automatic transmissions, a control has been hitherto proposed in which, during gear change operation, a hydraulic pressure applied to friction elements such as clutches or the like is stepwisely changed. That is, at an initial stage of the gear change operation, the hydraulic pressure is reduced for smoothing starting of engagement of the friction element, and thereafter, the hydraulic pressure is increased for shortening the period of the gear change operation, and at a final stage, the hydraulic pressure is reduced again for reducing a select shock produced upon completion of engagement of the friction element. That is, during the gear change operation, the hydraulic pressure for the friction element is temporarily increased. Increasing the hydraulic pressure starts when the engine speed is lowered from a maximum speed by a predetermined degree (for example, about 150 rpm). With this measure, the gear change operation is smoothly achieved in a shortened period.
However, due to its inherent construction, the above-mentioned conventional control is compelled to show a drawback when the vehicle speed is relatively low and/or any trouble occurs in the vehicle. That is, under such condition, it takes a long time to reduce the engine speed by the predetermined degree, and thus, completion of the gear change operation takes a considerably long time.